This research will elucidate the determinants of occupational disability and the interrelation between work and non-work factors using a U.S. Army data base. We will conduct a case-control study of severe knee injuries and discharge from the Army for knee-related disability nested within the cohort of all enlisted personnel on active duty in the U.S. Army between 1980 and 1997. We will conduct a series of comparisons to address three specific research questions: 1. What are the risk factors for severe knee injury? This analysis will identify occupational (e.g., duration of service, job type, physical demands) and personal characteristics (e.g., height, weight, sex, age) that increase the risk of knee injuries requiring lost work time and hospital treatment. The results from this analysis will identify pre-injury risk factors that operate both in the Army and in civilian occupations with similarly physically demanding jobs. By identifying pre-injury risk factors, intervention programs to reduce the risk of future injuries may be developed and evaluated. For this research question, we will compare soldiers hospitalized for knee injury to soldiers without knee-related hospitalizations. 2. What are the determinants of discharge from the U.S. Army for disabling knee injury? Studying all knee conditions severe enough to lead to disability discharge will necessarily identify factors that operate both before and after the injury. Pre-injury factors will increase the risk of injury. Post-injury factors relate to the likelihood of successful recovery and the likelihood of being granted disability benefits. As such, post- injury factors may include injury severity, physical fitness, having job demands amenable to modification to allow for recovery, and other, less measurable factors inherent in disability determinations. Cases for this research question comprise all soldiers discharged from service for a knee-related disability. Controls comprise soldiers not discharged from service for a knee-related problem. A direct comparison of risk factors for injury (question 1) and risk factors for disability (question 2) will separate pre- and post-injury factors. To elucidate which of the post-injury factors are related to recovery and which are related to disability determinations, question 3 is necessary: 3. What are the differences between injuries that lead to disability discharge and those that do not? Comparing individuals with severe knee injuries who are subsequently discharged for knee-related conditions with those who suffer similar injuries, but are not discharged, will allow for the identification of post-injury factors predictive of disability. This analysis will also clarify the roles of work and non-work factors in disability determinations. This research will be applicable to civilian populations to the degree that there are similarities in the more subjective components of military disability determinations and the granting of civilian insurance benefits. Synthesizing the answers to the research questions, above, will enable us to: Identify which risk factors for injury are also determinants of disability discharge Determine which risk factors for injury are not determinants of disability discharge Identify which determinants of disability discharge are not risk factors for injury.